REPRESSION OF THE L-ASPARAGINASE GENE DURING NODULE DEVELOPMENT IN LUPINUS-ANGUSTIFOLIUS

Upon the establishment of an effective nitrogen-fixing symbiosis in am ide-transporting plants the enzymatic activity and transcript levels o f L-asparaginase are dramatically decreased. This decrease in L-aspara ginase activity is essential for the correct functioning of the Rhizob ium-legume symbiosis in lupin in which asparagine, synthesized from re cently fixed nitrogen, is exported to aerial parts of the plant for us e in growth and development. Concomitant with this decrease in L-aspar aginase transcript a DNA-binding protein was detected in the nodules. This binding protein was not detectable in ineffective nodules, in nod ules treated with nitrate, or in root tips, mature roots, developing f lowers or developing seeds. The DNA-binding activity was shown to inte ract with a 59 bp sequence proximal to the transcription start site. W ithin this sequence a CTAAAAT direct repeat and a ACTGT/TGTCA incomple te inverted repeat were implicated in the binding of protein to the DN A by DNase I protection experiments. Competitive binding studies with synthesized binding sites were consistent with the CTAAAAT/TGTCA seque nce pair proximal to the transcription start site having the highest a ffinity for the DNA-binding protein. We postulate that this DNA-bindin g protein is associated with repression of L-asparaginase gene express ion in mature lupin root nodules.